Currently available replacements for D718/B688 bipolar transistors?

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BumpinYota

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Joined 2019
Greetings! First time poster and long time lurker/searcher!

I've executed multiple searches here and found many candidates to replace mentioned transistors but nothing I've found is currently available.

(These came out of a polk audio PA880 whose power supply was shorted and Ive since repaired.)

What currently is available out there?

b21aed8c2463ae05e9fe80357c0f9a4f.jpg


c81e09894f5af9c2bbd77029e786a56d_79a.jpg
 
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BumpinYota

Member
Joined 2019
Khron said:
Well, disregarding the fact that i seriously doubt those BJT's are part of a class-D amp (as per this subforum), how about MJW21195/6G parts?

MJW21196G ON Semiconductor | Mouser Finland
MJW21195G ON Semiconductor | Mouser Finland

Odds are good that stuff on Amazon / eBay, in the case of discontinued stuff, may well be fake.

Counterfeit Transistors
Click to expand...

My apologies as I had assumed being a mono amplifier that it was a class D. Upon further research I see that it isn't, but rather class AB.

Now those listed transistors (thank you for the suggestions :) ) are considerably "larger" than the old ones. I see the output capacitance is 500 pico-farads vs the origional's 280. Is this a concern that is addressed with larger gate resistors?

The origionals were being driven by a D600K and a B631k which also need replacement since one was a dead short across all leads. I believe I've found some suitable replacements but am not entirely sure. (863-MJE15030G and 610-MJE15031 both from mouser electronics.)

Would those be appropriate replacements?

Thank you again for your input it is greatly appreciated.
 
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Khron

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Joined 2005
khronscave.blogspot.com/
They're BJT's, so they don't have gates, first of all - they have bases. You might want to compare gains (hFE's) between the two, but they should be close enough that it won't really matter.

Remeber, these are part of a slow, linear class-AB amplifier - output capacitances and whatnot are damn near irrelevant.

Those MJE1503x's should be fine in those roles.
 
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BumpinYota

Member
Joined 2019
Khron said:
They're BJT's, so they don't have gates, first of all - they have bases. You might want to compare gains (hFE's) between the two, but they should be close enough that it won't really matter.

Remeber, these are part of a slow, linear class-AB amplifier - output capacitances and whatnot are damn near irrelevant.

Those MJE1503x's should be fine in those roles.
Click to expand...

Thank you for the info and input. I've placed all the parts on order, but I'm curious about the TL494 output on the power supply side.

My understanding is that the TL494 is supposed to generate an equal but 180* out of phase square wave from pins 5 and 6.

(At this writing I am out of state for a funeral) but if i recall correctly I am only getting a switching wave on pin 5. All power supply transistor bases have this switching wave as well with the same max, min, amd rms values.

Here is a pic of the switching wave im getting:
d7f10608cb3fb76862b25c648aea37ca.jpg



Here is a pic of the overall board layout and this IC is just peeking over the top of the transformer torroid.
An externally hosted image should be here but it was not working when we last tested it.
 
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